A bag contains $3$ red marbles, $10$ green marbles, and $8$ blue marbles. If a marble is randomly chosen, what is the probability that it is not blue?
Solution: There are $3 + 10 + 8 = 21$ marbles in the bag. There are $8$ blue marbles. That means $21 - 8 = 13$ are not blue. The probability is $ \frac{13}{21}$.